Heat generated in optically transparent materials resulting from laser action or nonlinear processes can create unwanted distortion of the light beams passing through the materials. Typically the generated heat is removed by contacting the material to a heat-removal agent, either another solid material, or a flowing liquid or gas. The resultant flow of heat leads to a temperature gradient in the material. Since the refractive index of a material is temperature dependent, the gradient in temperature leads to a gradient in the refractive index of the material.
The gradient in refractive index can lead to distortion of the beam passing through the material. The distortion typically consists of a modification of the phase across the beam, leading to a change in the beam properties. In some cases, especially if the material is inside an optical resonator, the distortion may be so significant that the resonator no longer functions, i.e. becomes “unstable.” Alternatively, the properties of the resonator may become so highly dependent on the amount of heat in the material that any device based on the resonator becomes impractical to use. For materials used as amplifiers, the beam distortion can lead to significant changes in the amplified output beam properties.
The present invention relates to an impending need to design optical systems that can function in the presence of thermally induced gradients in the index of refraction, in particular thermally induced gradients in nonlinear crystals.